1. Field of the Invention
The present invention relates to a duplexer wherein a surface acoustic wave filter chip is mounted on a multilayer package substrate. The present invention can be applied to, for example, an antenna RF (Radio Frequency) part employed in a mobile communication terminal such as a cellular telephone.
2. Description of the Related Art
There has heretofore been known a duplexer wherein a surface acoustic wave filter (e.g., a polarized surface acoustic wave filter) is mounted on a multilayer package substrate. Such a duplexer has been adopted in, for example, an RF part of a mobile communication terminal such as a cellular phone. In the mobile communication terminal, one antenna is employed in both transmission and reception. Therefore, a transmitting filter, a receiving filter, a branching line, etc. are mounted on one multilayer package substrate in the duplexer for the mobile communication terminal. The duplexer having the multilayer package substrate has been disclosed in the following Patent Documents 1 and 2, for example.
As shown even in FIG. 1 of the following Patent Document 1, the multilayer package substrate is made up of a plurality of package substrates in this type of duplexer. The top layer package substrate is provided on its upper surface with a chip of a surface acoustic wave filter. This chip is formed with, for example, a large number of comb-shaped filters. Wiring patterns are formed on the upper surfaces of the respective package substrates respectively. Further, the bottom layer package substrate is formed on its lower surface with signal electrodes, ground electrodes, etc. The surface acoustic wave filter chip is connected to top layer wiring patterns by, for example, wire bonding or the like. The wiring patterns of the respective layers and the electrodes (signal and ground electrodes) on the lower surface of the bottom layer are suitably connected to one another by through electrodes provided within the multilayer package substrate.
In addition, as shown in FIGS. 13 and 14 of the following Patent Document 2, there may be a case in which wiring patterns are provided on their corresponding side surfaces of the multilayer package substrate, and electrodes provided on an upper surface of a top layer package substrate and electrodes provided on a lower layer of a bottom layer package substrate are connected to one another.
The duplexers having such configurations are effective in bringing the mobile communication terminal into less size and weight.
Patent Document 1                Japanese Unexamined Patent Publication No. 2002-124847 (3rd and 4th pages and FIG. 1)        
Patent Document 2                Japanese Unexamined Patent Publication No. Hei 10(1998)-126213 (3rd page and FIGS. 13 through 17)        
As described above, one antenna is used in both transmission and reception in the mobile communication terminal or the like. Assuming that a transmit frequency band ranges from 824 MHz to 849 MHz and a receive frequency band ranges from 869 MHz to 894 MHz, a surface acoustic wave filter in which the range of 824 MHz to 849 MHz is defined as a pass band, and other frequencies are defined as an attenuation band, and a surface acoustic wave filter in which the range of 869 MHz to 894 MHz is defined as a pass band, and other frequencies are defined as an attenuation band, are formed on the above chip. This chip is mounted on its corresponding upper surface of the top layer package substrate.
As to the duplexing characteristics here, pass band characteristics in the pass band and attenuation characteristics in the attenuation band must meet predetermined specs. However, the surface acoustic wave filter changes in characteristic when it is formed as the unitary chip and after being mounted on the multilayer package. Thus, the characteristic of the singular chip must be determined in anticipation of this change in characteristic. As one factor of the change in characteristic, may be mentioned an influence of inductance of each of bonding wires for connecting the chip and wiring patterns. As other factors, may be considered, imperfection of a ground potential portion provided within the multilayer package, and coupling between the wiring patterns.
Therefore, the design conditions of the respective surface acoustic wave filters have heretofore been fixed up in consideration of the inductance of each bonding wire. However, such a method has encountered difficulties in obtaining high-performance characteristics as shown as design characteristics.